1. Field of the Invention
The present invention relates to a steel sheet for dissimilar materials weldbonding to an aluminum material and a dissimilar materials bonded body formed by weldbonding the steel sheet to the aluminum material, which can have a high bonding strength.
2. Description of the Related Art
In recent years, it has been attempted to apply a light alloy such as an aluminum system material (a generic name of pure aluminum and aluminum alloys, hereunder referred to simply as an aluminum material) to a structure such as an automobile from the viewpoint of environmental issues.
From the viewpoints of weldability, corrosion resistance, and formability however, aluminum materials are used only for some of automobile members. In this light, steel sheets for automobiles which have heretofore been used and aluminum materials are in a coexistent state and the application of Fe—Al dissimilar materials bonding to a member formed by combining a steel sheet with an aluminum material has increasingly been demanded.
A problem in Fe—Al dissimilar materials bonding is that an intermetallic compound layer of Fe and Al which is hard and very brittle is formed at a bonded interface. Consequently, there are many cases where a sufficient bonding strength is not secured due to the formation of such a compound layer even when the materials are apparently bonded.
In consideration of the above situation, bonding with bolts, rivets, or those combined with adhesives has heretofore been adopted for the bonding of a dissimilar materials bonded body (a dissimilar metals member). However, such a bonded joint has the problems of reliability, airtightness, costs and others.
In this light, methods of spot-welding such a dissimilar materials bonded body have heretofore been variously studied. For example, methods of inserting an aluminum-steel clad material between an aluminum material and a steel material are proposed (refer to Patent Documents 1 to 6). Further, methods of plating the side of a steel material with a metal having a low melting point or inserting a metal having a low melting point are proposed (refer to Patent Documents 7 to 9). Furthermore, a method of interposing insulator particles between an aluminum material and a steel material (refer to Patent Document 10) and a method of forming jogs on a member beforehand (refer to Patent Document 11) are proposed.
In addition, also proposed is a method of removing a nonuniform oxide film on an aluminum material, thereafter heating the aluminum material for 8 hours at 200° C. to 450° C. in the air and thus forming a uniform oxide film, and, in the state wherein the contact resistance of the aluminum surface is increased, applying spot welding while an aluminum-steel double-layer steel sheet is used as the insertion material (refer to Patent Document 12).
In the meantime, it is publicly known that, when elements such as Si, Mn, Al and others which tend to form oxide are added in order to increase the strength of a steel sheet, oxide containing such Si, Mn, Al and others forms on the surface of the base material. Then, it is also publicly known that the oxide containing such Si, Mn, Al and others hinders the adhesiveness between a surface coating such as a zinc coating and a steel sheet. On the contrary, it is also publicly known that, by subjecting the steel sheet to pickling or the like and thereby controlling the thickness of an oxide layer containing such Si, Mn, Al and others in the range from 0.05 to 1 μm, the adhesiveness between the surface coating such as a zinc coating and the steel sheet and spot-weldability between steel sheets improve (refer to Patent Document 13).
[Patent Document 1] JP-A No. 55066/1992 (full text)
[Patent Document 2] JP-A No. 127973/1992 (full text)
[Patent Document 3] JP-A No. 253578/1992 (full text)
[Patent Document 4] JP-A No. 111778/1993 (full text)
[Patent Document 5] JP-A No. 63763/1994 (full text)
[Patent Document 6] JP-A No. 178563/1995 (full text)
[Patent Document 7] JP-A No. 251676/1992 (full text)
[Patent Document 8] JP-A No. 24581/1995 (full text)
[Patent Document 9] JP-A No. 14383/1992 (full text)
[Patent Document 10] JP-A No. 228643/1993 (full text)
[Patent Document 11] JP-A No. 174249/1997 (full text)
[Patent Document 12] JP-A No. 63763/1994 (full text)
[Patent Document 13] JP-A No. 294487/2002 (full text)